So what was Rocks doing that was more important than fighting Roger, and what would it be that Roger would try and stop him? And what would it be that even Garp would change from going after Roger to Rocks?

If I had to guess I'd say Rocks was going to kill Garling's kids as revenge for them trying to kill his kids, Roger got in his way, Dragon was protecting the kids like the mother begged, and Garp seethed that Rocks would go after his son who was protecting babies and swapped his target to Rocks.

We already know that Rocks has no problems attacking children from him stabbing Loki, and "sins of the father can't be attributed to the child" is a big theme with Roger, Whitebeard, and Garp.